One thing you might consider is a singlespeed or 3/7/8/11sp internal gear hub out back. One of the components most prone to rust is the chain, there are also steel components on derailleurs which will rust as well. KMC (chain manufacturer) makes a chain model S10, which is stainless steel, another one of their chains is specifically marketed with "rust buster" coating over a plain steel chain. IGH drivetrain with either of these chains will be the most rust free.